Resolving the interfacial behaviour in complex magnetic nanostructures

Sean LangridgeRutherford Appleton Laboratory, STFC

sean.langridge@stfc.ac.uk

OSNS16

Alternatively…

God made solids, but surfaces were the work of the devil!Wolfgang Pauli 1900-1958

en.wikipedia.org

Alternatively…

"There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics“

RP Feynman 1918-1988

en.wikipedia.org

Grand Challenges: Nanomagnetism

Grand Challenges

Spin-Based Qubits

Magnetic Logic

Computer From a Test Tube

Control of pure non-local spin

currents

Instant Boot Computer

Electronic control of

magnetism

R.T. Magnetic Semiconductors

Adapted from S.D. Bader Rev Mod Phys 78 (2006)

(Super-)Spintronics

Control and manipulation of the spin degree of freedom in the solid state environment

SpinTransportDynamicsRelaxation

Information storageQuantum Computing

Quantum Well StateBand matching important e.g. Fe/Cr

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Co/Cu GMR

A history of storage/spintronics

1956RAMAC

1988Discovery of GMR

1991Spin Valve (IBM)

1994MTJ

1997 SV in a HDD

2004Epitaxial MTJ

2005MTJ Read Heads

20064Mb MRAM

Chappert, Fert, Nguyen van Dau Nat. Matl. 6, 813 (2007)

The Importance of interfaces

S.S.P.Parkin et al. Science 520 5873 (2008)

Awschalom and Flatté Nat. Phys. 3 153 (2007)

j

Ramesh and Spaldin Nat. Mat. 6, 21 (2007)

Maccherozzi et al. Phys. Rev. Lett. 101, 267201 (2008)

I. N. Krivorotov et al., Science 307, 228 (2005).

Relevance of scattering techniques to interfacial effects

Adapted from I.K. Schuller et al. J. Magn. Magn.Mater. 200 (1999) 571.

Domain Structures

Exchange BiasConventional EXBSynthetic EXBFrozen magnetism

Singlet-Triplet Superconductivity

Heusler, DMS

Frustration

Organic Spintronics

Interfacial Magnetism

Nanoscale Electronic Phenomena Research

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Surface Magnetism

To understand and control spin and interfacial phenomenaProbe length scale (<1nm to >1000nm)Vector Magnetometry ~<0.1µB per f.u.

Helimagnetism-Skyrmions

AF Semiconductors

Motivation & OutlineMotivation

Unique and quantitative description of complex electronic materials on the microscopic lengthscaleRelating the functional properties of materials to their atomic and nanoscale structure

Introduction to Polarised Neutron Reflectivity (PNR)Ferrimagnetic insulators for spintronics/magnonics

Understand interfacial behaviour in spin-current, magnonic systemsUnderstand some of the low-T anomalies in thin ferrimagnetic insulator filmsCharacterise the spin axis on FI/AFI systems

Chiral MagnetismControl of helical structuresStudy of DMI in thin films

Magnetocaloric MaterialTowards Super-SpintronicsSmall Angle ScatteringSummary & Conclusions

Acknowledgements

B. J. HickeyA. MitraO. CespedesN. PorterC.S. Spencer

C.J. KinaneN-J. SteinkeJ.F.K. CooperD Alba VeneroA. Caruana

T. Zhu

S. SinghS. Basu

NEUTRON REFLECTIVITYRecall Webster Talk Last Week

PNR from a single layer

Collins-McIntyre, L. J. et al.Europhysics Lett. 115, 27006 (2016)

Non spin flip++ measures b + Mz- - measures b - Mz

Spin flip+ - measures Mx + iMy- + measures Mx - iMy

By fitting all components the direction and strength of the magnetic moment can be measured as a function of depth

1D- Polarisation Analysis

PolREF BEAMLINETOF wavelength band 1Å – 16Å

Vertical and horizontal geometry

Non-polarised, polarised and polarisation analysis modes

Sample point goniometer capable of moving 1000kg

Experimental Setups:

• GMW Magnet (± 1T).• Cryostat (2.5K -300K), (sub

1K fridge) ad hoc in-situ transport.

• Vacuum furnace (300K –800K).

• PNR/PA Polarised modes.• Various soft matter setups• H loading

Generating ‘Pure’ Spin currents: FerrimagneticInsulators

Understanding the interfacial behaviour

‘As spin pumping in heterostructures requires the transfer of spin information across an interface, the role of interface quality must be understood thoroughly, yet itremains unclear.’

Gray, M. T. et al. Phys. Rev. Appl. 9, 064039 (2018).

Meet the Hall family…

For a century only the two effectsExtrinsic vs. intrinsic

SOC impuritiesE.g. through spin injection

Ideal for sensor technologies to low-power technologies

Spin transistors, spin logic, and spin quantum computing

Chang, C.-Z. & Li, M. Quantum anomalous Hall effect in time-reversal-symmetry breaking topological insulators. J. Phys. Condens. Matter 28, 123002 (2016).

‘On a new action of the Magnet on Electric Currents’ American Journal of Mathematics vol 2, 1879, p.287-292

Generate a spin imbalance

Various mechanisms

Žutić, I. & Dery, H. Spintronics: Taming spin currents. Nat. Mater. 10,647 (2011)

Qiu, Z., Hou, D., Uchida, K. & Saitoh, E. Influence of interface condition on spin-Seebeck effects. J. Phys. D. Appl. Phys. 48,164013 (2015).

Inverse Spin Hall effectExploit the SOC

Spin and orbital motion are coupled

SOC limits spin diffusion lengthChannel for spin relaxation

SOC makes effective sink for the angular momentumApplying a field increases the number of magnons and the lattice acts as a source of spin currentUse YIG as a source of spin currentPt as a spin converter

Žutić, I. & Dery, H. Spintronics: Taming spin currents. Nat. Mater. 10, 647 (2011)

YIG: Y3Fe5O12

YIG ferrimagnetic insulatorFerrimagnetic insulator

Bandgap 2.58eVTc = 560K

Transparent above 600nmLow absorption in IRVery low dampingSmall linewidth for esrLong spin wave decay length/ magnon damping

Ideal for:optical and magneto-optical applications, microwave filtersSpin Seebeck/Peltier applications

Jungfleisch, M. B. et al. Thickness and power dependence of the spin-pumping effect in YIG/Pt Phys. Rev. B 91, 134407 (2015).

• Princep, A. J. et al. The full magnon spectrum of yttrium iron garnet. npj Quantum Mater. 2, 63 (2017).

Thin film YIGNumerous Growth techniquesRF sputtered followed by anneal 850c for 2 hrsGrow on GGG (Gd3Ga5O12)0.06% lattice mismatchSuperSTEM

Abberation correctedHAADF6nm diffusion Diffusion co-efficient 1x10-17cm2s-1

(agrees with bulk)

PNR DataAnalysed within a simple 2-layer model

2 layer model Pristine YIGGd doped YIG3.8µB/u.c.

Gd 6-7nm diffusionExcellent agreement with x-ray and SuperSTEMGd absorption cross-section enhances sensitivityGd Iron Garnet: room temperature compensation

80nm

Temperature DependenceGd Iron Garnet: room temperature compensationM(T) YIG follows Bloch lawAntiparallel moment develops at low temperature near to interfaceGd substitutes on Y siteUnusual magnetisation temperature dependence from Gd diffusionEffect on low-T FMR linewidthISHE correlates with interface quality

Probably not the full story…

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z (Å)

250K 150K 80K 50K 5K

Mean Field Model

No observable interfacial moment

XMCD: 0.003µB averagedEffect on low-T FMR linewidthReduced Spin SeebeckPossibilities to lower growth temperature to limit diffusionISHE correlates with interface quality Relevance to spin pumping, spin torque investigations

S. Gebrägs et al. Appl. Phys. Lett. 101, 262407 (2012);

MECHANISM FOR THE –VE SPIN HALL MR

Spin Hall Magnetoresistance (SMR)SHE

Conversion of electric current into a transverse spin currentGenerates spin current and accumulation

How can we control/observe this?

Ferr(i)omagnetic insulator Interfacial spin mixingPt film resistance depends on the YIG orientation

SMR

Nakayama, H. et al. Spin Hall Magnetoresistance Induced by a Nonequilibrium Proximity Effect. Phys. Rev. Lett. 110, 1–5 (2013).

SMRExperimentally verified in:

Pt/YIG, Ta/YIG, W/CoFeB

Pt(4)/NiO(1-2)/YIG(10)/GGGSpin current enhanced if an AF layer is inserted (NiO)

High spin transfer efficiencySpin current mediated by magnons

Expect +ve SMRCharacteristic temperature (below TNéel)Changes sign at low temperature

Spin flip scattering Inversion of Jeadd PRL 118, 067202 (2017)Spin flop coupling PRL 118, 147202 (2017)

– Spin axis of NiO MYIG

Wang, H., Du, C., Hammel, P. C. & Yang, F. Antiferromagnonic Spin Transport from YIG into NiO Phys. Rev. Lett. 113, 097202 (2014).

GGG/YIG(15)/NiO(2)/Pt(4nm)PLDRT, 9.7798GHzResonance field comparable to bulkM = 145emu/cm3

+ve SMR@RTChange in sign at ~210K

Hou, D. et al. Tunable Sign Change of Spin Hall Magnetoresistance in Pt/NiO/YIG Structures. Phys. Rev. Lett. 118, 147202 (2017).

Expect +ve SMRCharacteristic temperature (below TNéel)Changes sign at low temperature

Spin flip scattering Inversion of Jeadd PRL 118, 067202 (2017)Spin flop coupling PRL 118, 147202 (2017)

– Spin axis of NiO MYIG

Structural Profile

Structural profile from PNRPt (4 nm)/NiO (2 nm)/YIG/GGGAgreement with XRR & AFM

Evidence for uncompensated moment?

Induced Pt moment (proximity/accumulation)

Spin accumulation at the interfaces

Sensitivity to <0.05µB/PtLess than 0.02µB/Pt (within 1XMCD: 0.003µB averaged

S. Gebrägs et al. Appl. Phys. Lett. 101, 262407 (2012);

Parallel Alignment

Does not describe SF data

Interfacial NiO layer

Promising…

Orthogonal YIG

Pt (4 nm)/NiO (2 nm)/YIG/GGGSignificant Spin flip scatteringNiO Quantization axis orthogonal to YIGUncompensated moment in NiO

Domain state modelEB at low temperature

Nowak, U. et al. Phys. Rev. B 66, 014430 (2002)

Small Angle Scattering

A. Wiedenmann J. Appl. Cryst. (2000). 33, 428-432

Grigoriev et al. Phys Rev B 100, 094409 (2019)

Mühlbauer, S. et al. Magnetic small-angle neutron scattering. Rev.

Mod. Phys. 91, 015004 (2019).

Grazing Incidence to give depth selectivityDifference gives the interference term

Summary & OutlookThe neutron possess a range of characteristics well matched to both curiosity driven and technologically relevant nanoscale research

Absolute, quantitative results provide a robust test of theoryFrom atomic and nano through to micro-macroscopic lengthscales

Examples drawn from:Ferromagnetic InsulatorsChiral MagnetismMagnetocaloricBiomedical applications, Clean Energy

Frontier ResearchThin film quantum matterNovel sample environmentSmaller SamplesIncreasing connection with calculation/theory